Method of making graphene layers, and articles made thereby

ABSTRACT

There is provided a method for forming a graphene layer. The method includes forming an article that comprises a carbon-containing self-assembled monolayer (SAM). A layer of nickel is deposited on the SAM. The article is heated in a reducing atmosphere and cooled. The heating and cooling steps are carried out so as to convert the SAM to a graphene layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/278,344, filed on Oct. 21, 2011, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to thin film fabrication techniques, and moreparticularly to techniques for making thin films of carbon.

ART BACKGROUND

Graphene is a form of carbon having a hexagonal planar atomic latticestructure, as shown schematically in FIG. 1. Graphene has attracted agreat deal of interest since 2004, when researchers found a simple wayto produce isolated atomic planes of graphene and demonstrated a numberof unique physical properties of such planes. The term “graphene” isoften used in reference to atomic monolayers. Herein, however, we usethe term more generally to apply to thin carbon films that have thegraphene lattice structure and are thin enough, typically five or feweratomic layers thick, that their electronic properties are dominated bythe effects of reduced dimensionality and are therefore distinct fromthe electronic properties of bulk graphite.

Graphene is known to exhibit semiconductive or semimetallic electronicproperties, as well as a direct band gap and high electron mobility. Forthat reason, among others, graphene is viewed as a candidatematerial-for transistors and integrated circuits.

Difficulties remain, however, in the production of graphene ofsufficient quality and quantity, at low enough cost, to make its use inelectronic devices commercially feasible.

The known techniques for making graphene films include mechanicalexfoliation of graphite, epitaxial growth on suitable substrates,reduction from graphite oxide, precipitation from solutions of carbon inmolten metal, and reduction of monomers such as sucrose or polymers suchas polymethyl methacrylate (PMMA) on suitable substrates.

There is still a need, however, for a method of graphene synthesis thatcan form large films of uniform, controllable thickness at costs thatare low enough to make this material industrially relevant.

SUMMARY OF THE INVENTION

I have found a new approach to producing graphene films, in whichself-assembled monolayers (SAMs) are used as the carbon source forgraphene synthesis. A SAM is formed when a monolayer of surfactantmolecules, or other amphiphilic molecules, is adhered to a substratesurface by bonding between the substrate and a head end of eachmolecule. The tail ends of the amphiphilic molecules, which extend awayfrom the substrate, align with each other to create a layer whose carboncontent is determined by the chain length and other factors that dependon the particular choice of chemical species. As a consequence, thethickness of the resulting graphene film, which depends on the arealdensity of carbon in the source material, may be controlled by selectingthe composition of the source material.

Accordingly, our approach in one embodiment comprises forming an articlethat comprises a carbon-containing SAM, depositing a layer of nickel onthe SAM, heating the article in a reducing atmosphere, and cooling thearticle. (The term “cooling” is meant broadly and would include, amongother techniques, purely passive cooling by simply removing the articleto an environment of lower temperature, as well as reducing the ambienttemperature according to a programmed schedule.) The heating and coolingsteps are carried out so as to convert the SAM to a graphene layer.

In an embodiment, the article comprising a SAM is formed, at least inpart, by depositing an amphiphilic compound on a substrate. Theamphiphilic compound comprises a head group and a tail group, the headgroup comprises a moiety that adheres to the substrate, and the tailgroup comprises a hydrocarbon moiety.

In an embodiment, the substrate comprises silicon dioxide, thehydrocarbon moiety comprises at least one aliphatic chain, and the headgroup is a silyl ester functional group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the atomic arrangement in agraphene monolayer.

FIG. 2 is a schematic depiction of one unit of a SAM at the molecularlevel.

FIG. 3 is a flowchart of an illustrative process for synthesizing agraphene layer according to an embodiment of the invention.

FIG. 4 is a schematic drawing illustrating progressive stages of theprocess of FIG. 3.

FIG. 5 is a schematic drawing illustrating the doping of a graphenelayer.

FIG. 6 is a schematic drawing illustrating progressive stages of aprocess for forming a patterned graphene layer.

DETAILED DESCRIPTION

As illustrated in FIG. 2, a self assembled monolayer is an organizedlayer of amphiphilic molecules 200 which are oriented with respect to asubstrate 210 due to a specific physical or chemical interaction withthat substrate. Typical SAM molecules are surfactant molecules with along aliphatic chain 220, which we refer to as a “tail” group, and afurther portion 230, referred to hear as a “head” group, withfunctionality specific for surface binding. Some exemplary functionalgroups having the ability to bind to certain surfaces are thiol groups,carboxylic acid groups, and silyl ester groups. Silyl esters, inparticular, can form covalent bonds with silicon oxide surfaces, such assilica glass surfaces. (As used herein, the terms “silica”, “siliconoxide”, and “silicon dioxide” are interchangeable, and should beunderstood broadly as encompassing all materials commonly meant by anyof those names.)

Silyl esters are of particular interest for the synthesis of graphenelayers because they can bond to silica glass substrates, and becausethey can be readily obtained or readily synthesized with any desiredaliphatic chain length Cn (n denotes the number of carbon atoms) withina broad range. The range C6-C18, in particular, is of current interestfor graphene synthesis. The precursor typically used to deposit silylester monolayers is an alkyl trichlorosilane, although trialkoxysilanescan also be used.

Accordingly, the SAM may be made of any of various amphiphilicmaterials, provided that such materials will adhere to the desiredsubstrate surface. Although compounds having long aliphatic chains arecurrently of primary interest, aromatic compounds may also be useful inthis regard.

For adhesion to silicon oxide substrates, alkyltrialkoxysilanes of Chainlength C6-C18 are currently of primary interest. However, many othercompounds may be useful, particularly when other substrate materials areconsidered. For example, SAMs on gold surfaces may be formed fromalkanethiol, dialkyl disulfide, dialkyl sulfide, alkyl xanthate, anddialkylthiocarbamate compounds.

An illustrative process for synthesizing a graphene layer will now bedescribed with reference to the flowchart of FIG. 3 and the schematicdrawings of FIG. 4. Reference numerals 300-330 refer to featuresillustrated in FIG. 3. Reference numerals 400-440 refer to featuresillustrated in FIG. 4.

At 300, a monolayer of an amphiphilic compound, exemplarily an alkyltrichlorosilane of chain length C6-C18 is formed on the surface of asilica glass substrate by thermal evaporation, by microcontact printing,or by another appropriate transfer technique. The monolayerself-assembles to form a SAM.

The monolayer is deposited under conditions that assure that it iscomplete, and that it is ordered with respect to the substrate. In thecase of alkyl trichlorosilanes, a typical procedure involves theexposure of the substrate (typically silicon dioxide on silicon) to ahumid atmosphere to encourage hydration of the silicon dioxide surface.The hydrated silicon dioxide surface is then treated with thealkyltrichlorosilane by dipping of the substrate in a bath ofalkyltrichlorosilane in hydrocarbon solvent (typically hexanes ortoluene) and removing the substrate. Water from the hydrated silicasurface permits the hydrolysis of the trichlrosilane which results inbonding of the silyl group with the silicon dioxide surface. Furtherhydrolysis of remaining chlorosilane functionality permits bondingbetween the neighboring silyl groups to create the Si—O—Si bonds. As aconsequence, the resulting SAM is bonded to the silicon dioxide surfaceand also bonded across the film through the Si—O—Si bonds betweensurfactant molecules.

Numerous appropriate amphiphilic compounds have been reported in thescientific literature, together with the conditions for them to formSAMs on appropriate substrates. Reference may be usefully made, forexample, to A. Ulman, “Formation and Structure of Self-AssembledMonolayers,” Chem Rev. (1996) 1533-1554 and the references citedtherein. Accordingly, appropriate conditions will be well known to thoseskilled in the art, and need not be discussed in detail herein.

At 310, a layer 410 of nickel, overlying the organic monolayer, isformed by thermal evaporation. Nickel is known to be a catalyst forconverting organic carbon into graphene or graphite. A typical thicknessfor the nickel layer is in the range 50-500 nm. In an alternateprocedure, the nickel may be deposited electrochemically rather than byevaporation.

At 320, the SAM is converted to graphene 420 by heating with catalysisby nickel. From earlier studies of the catalytic conversion of organiccompounds, I believe that an effective temperature range for theconversion is 800-1000.degree. C. The conversion takes place in areducing atmosphere consisting of flowing argon and hydrogen.

In this regard, reference is usefully made to Z. Sun, et al., “Growth ofgraphene from solid carbon sources.” Nature 468 (25 Nov. 2010) 549-552,and to the supplementary information thereto, which is available on-lineat www.nature.com. Sun et al. reported on their growth of graphene fromPMMA films spin-coated onto 25 micrometer thick copper foil. A coppersubstrate was selected because it was known to catalyze the formation ofgraphene. A typical procedure, as reported by Sun et al., was carriedout in a 1-inch quartz tube furnace evacuated to 100 mTorr andmaintained at a temperature of 1000.degree. C. The coated foil wasintroduced into the furnace with a hydrogen flow of 50 sccm and an argonflow of 500 sccm for 10-20 minutes, while maintaining the total pressurebelow 30 Torr. (An “sccm” is a standard cubic centimeter per minute.)The copper foil was then removed from the hot zone of the furnace andrapidly cooled to room temperature under a hydrogen-argon atmosphere.

Sun et al. reported that the thickness of the resulting graphene layercould be controlled by controlling the gas flow rates. A bilayer wasobtained at 1000.degree. C. with an argon flow of 500 sccm and ahydrogen flow of 10 sccm. Reducing the hydrogen flow to 3-5 sccm causedgreater numbers of layers to form. Increasing the hydrogen flow to 50sccm or more resulted in the formation of monolayers only. High qualitygraphene monolayers were obtained at process temperatures as low as800.degree. C.

Sun et al. reported that they also tested nickel, among other materials,to determine whether it would be an effective substrate for growinggraphene. They found that nickel is an efficient catalytic substrate.

We believe that the process conditions reported by Sun et al.,particularly at the low end of their temperature range, i.e. at800.degree. C., Would be an effective starting point for optimizing ournew process. We believe that from such a starting point, optimizationcould be achieved without undue experimentation.

At 330, the nickel layer is removed, exemplarily by an iron (III)chloride (FeCl.sub.3) oxidative etch, leaving behind article 430, inwhich graphene layer 420 overlies substrate 440. Alternatively, thenickel may be removed by dissolving it in hydrofluoric acid (HF). HF mayalso be used to remove the underlying silicon dioxide substrate (whichmay itself be a layer deposited on a base member) to yield afreestanding graphene film.

It will often be desirable to provide a graphene layer that has beendoped with an electron donor such as nitrogen or an electron acceptorsuch as boron, so that a semiconductive layer with specified propertiescan be formed.

In one doping technique, amphiphilic compounds suitable for SAMformation are provided, which include boron-containing ornitrogen-containing surfactant or other amphiphile molecules. Manycompounds which will readily form appropriate substituents in, e.g.,aliphatic hydrocarbon chains are known. Examples include borate esters,boronic acids, and amines. The doping concentration in the resultantgraphene layer may be controlled by to controlling the relative amountof alkyl surfactant and nitrogen-containing or boron-containingsurfactant in the SAM layer.

Upon conversion of the SAM to graphene, I believe that an effectivefraction of the dopant atoms will be incorporated in the graphenelattice in substitution for lattice carbon atoms. Support is found, forexample, in Z. Sun et al., cited above, which reports successfulnitrogen doping of graphene films converted from PMMA by addingmelamine(1,3,5-triazine-2,4,6-triamine) to the spin-deposited PMMAsolution. As reported there, the doped graphene film was grown byheating in a tube furnace at 1000.degree. C. for 10 minutes at oneatmosphere pressure, with 100 sccm hydrogen flow and 500 sccm argonflow. The higher pressure was believed necessary in order to maintainthe nitrogen-atom concentration. X-ray photoemission spectroscopyconfirmed that nitrogen atoms were incorporated in the graphene lattice.

Patterning of the dopant in the resulting graphene layer is highlydesirable. The SAM can be formed with a pattern that includes areaswhich are without dopant atoms and other areas which includedopant-containing compounds at various concentrations. These areas canbe defined by various patterning techniques e.g., microcontact printing.

For p-type doping, for example, a conventional surfactant used to formmonolayers is mixed with a desired concentration of a further surfactantwhose molecular formula includes boron (or aluminum), and for n-typedoping, the conventional surfactant is mixed with a nitrogen-containingsurfactant (alternatively, a phosphorus-containing surfactant). Thesurfactant mixture is provided as a microcontact printing ink which wetsa silicone elastomeric stamp, after which it is transferred from thestamp to selected domains on the substrate surface. The inked domainsare bound to the substrate surface by hydrolysis.

By way of illustration, one possible ink that may be used for p-typedoping is a mixture of tris(trimethylsilyl) borate and alkyltrichlorosilane. One possible ink that may be used for n-type doping isa mixture of aminohexyl triethoxysilane and alkyl trichlorosilane.

When the SAM is subsequently converted to graphene, at least some of theboron atoms from the p-type ink or nitrogen atoms from the n-type inkbecome incorporated in the graphene lattice in substitution for carbon.This is illustrated in FIG. 5, where “B” indicates a boron atomincorporated in the lattice for p-type doping, and “N” indicates asimilarly incorporated nitrogen atom for n-type doping.

In some cases it will be desirable to pattern the doped graphene layers,for example by providing dopant concentration gradients having specifiedprofiles, or by forming semiconductor junctions by adjoining p-dopedfeatures to intrinsic or n-doped features, or similarly by adjoiningn-doped features to intrinsic features. The formation of semiconductorjunctions and dopant gradients would be useful, for example, for makingdiodes, field-effect transistors, and other electronic devices.

Patterned SAMs may be provided by any of various known techniques, notleast of which is microcontact printing as illustrated, e.g., in FIG. 6.Applications of microcontact printing for the patterning of SAMs aredescribed, for example, in Y. Xia et al., “Soft Lithography,” Amer. Rev.Mater. Sci. 28 (1998) 153-184, and references cited therein.

Accordingly, a patterned SAM having a first type of doping provided asdescribed above may be deposited on substrate 600 in a first transferstep by microcontact printing using patterned stamp 610 to transfer ink620 to the substrate. The first transfer step is spatially selective, sothat some domains 630 on the substrate surface will be coated, and otherdomains will remain bare. Then, in one or more further transfer steps,patterned SAM layers having different dopant types or different dopantlevels may be deposited on selected parts of the bare domains.

The further transfer steps may be performed using microcontact printing.However, portions to which SAMs have already been bound will rejectfurther applications of the amphiphilic compounds that are SAMprecursors. Accordingly, the SAMs that have already been formed mayinherently form a mask for aligning the next transfer step. In such acase, the next application of the amphiphilic precursor compound may bemade in a spatially non-selective way, e.g. by dipping or by vaporexposure.

After the complete layer of patterned SAM domains has been formed,conversion to graphene is performed as described above.

As explained above, the growth of graphene 650 is catalyzed by a layer640 of nickel that is deposited over the SAM. The layer of nickel mayalso be patterned, e.g. by deposition through a mask or by lithographicetching. During the subsequent heat treatment, only those SAM portionsthat lie beneath the patterned nickel layer will be converted tographene. This provides a further method for patterning the graphenelayer.

What is claimed is:
 1. A method comprising: depositing a layer ofamphiphilic compounds having two or more different chemical compositionsonto a substrate; allowing the layer to self-assemble into aself-assembled monolayer (SAM) onto two or more regions of thesubstrate, each region including one of the amphiphilic compounds; andheating the SAM layer to convert it to a layer of grapheme, wherein atleast one of the amphiphilic compounds comprises an n-type or p-typedopant element, and at least one p-n, p-i, or n-i semiconductor junctionis formed between the grapheme regions.
 2. The method of claim 1,wherein the depositing step includes transferring at least one of theamphiphilic compounds onto the substrate by microcontact printing.
 3. Anarticle formed by the method of claim
 1. 4. The method of claim 1,further comprising: applying nickel on the SAM layer.
 5. The method ofclaim 1, wherein the depositing step is spatially selective on thesubstrate.
 6. The method of claim 5, further comprising: depositing asecond layer of amphiphilic compounds having two or more differentchemical compositions onto the substrate.
 7. The method of claim 1,further comprising: removing the substrate from the graphene layer. 8.The method of claim 7, wherein a second layer is deposited directly onthe substrate adjacent regions including the SAM layer.
 9. The method ofclaim 7, wherein the depositing step is spatially non-selective.
 10. Themethod of claim 1, wherein the amphiphilic compounds are selected fromboron-containing surfactants, nitrogen-containing surfactants, andcombinations thereof.
 11. The method of claim 10, wherein theamphiphilic compounds are selected from borate esters, boronic acids,amines and combinations thereof.
 12. The method of claim 1, wherein atleast one of the amphiphilic compounds comprises an n-type dopantelement, at least one of the other amphiphilic compounds comprises ap-type dopant element, and at least one p-n semiconductor junction isformed between the graphene regions.
 13. A method comprising: depositinga layer of amphiphilic compounds having two or more different chemicalcompositions onto a substrate; allowing the layer to self-assemble intoa self-assembled monolayer (SAM) onto two or more regions of thesubstrate, each region including one of the amphiphilic compounds; andheating the SAM layer to convert it to a layer of graphene, wherein atleast one of the amphiphilic compounds comprises an n-type or p-typedopant element, and at least one dopant gradient is formed between thegraphene regions.
 14. The method of claim 13, wherein at least one p-n,p-i, or n-i semiconductor junction is formed between the graphemeregions.
 15. The method of claim 14, wherein at least one of theamphiphilic compounds comprises an n-type dopant element, at least oneof the other amphiphilic compounds comprises a p-type dopant element,and at least one p-n semiconductor junction is formed between thegraphene regions.
 16. The method of claim 13, wherein the depositingstep includes transferring at least one of the amphiphilic compoundsonto the substrate by microcontact printing.
 17. A method comprising:depositing a layer of amphiphilic compounds having two or more differentchemical compositions onto a substrate; allowing the layer toself-assemble into a self-assembled monolayer (SAM) onto two or moreregions of the substrate, each region including one of the amphiphiliccompounds; an heating the SAM layer to convert it to a layer ofgraphene, wherein the amphiphilic compounds are selected fromboron-containing surfactants, nitrogen-containing surfactants, andcombinations thereof.
 18. The method of claim 17, wherein at least oneof the amphiphilic compounds comprises an n-type or p-type dopantelement, at least one p-n, p-i, or n-i semiconductor junction is formedbetween the graphene regions, at least one of the amphiphilic compoundscomprises an n-type dopant element, at least one of the otheramphiphilic compounds comprises a p-type dopant element, and at leastone p-n semiconductor junction is formed between the graphene regions.19. The method of claim 17, wherein at least one of the amphiphiliccompounds comprises an n-type or p-type dopant element, and at least onedopant gradient is formed between the graphene regions.
 20. The methodof claim 17, the amphiphilic compounds are selected from borate esters,boronic acids, amines and combinations thereof.